This invention relates generally to the field of generating very short wavelength radiation and relates specifically to a method and an apparatus for generating very short wavelength radiation which is monochromatic, highly directional and tunable throughout a region spanning multiple orders of magnitude in wavelength, or equivalently, energy.
As is well known, the ability to direct a specified beam of radiation at a particular structure allows an observer to gain information about that structure. Visible light allows the reception of information about objects, with the human visual apparatus. Illuminating a structure with other radiation, such as x-rays, having a wavelength of less than 10.sup.-8 m, can provide information about structures that are not accessible using visible light. One reason is that x-rays pass through structures that are opaque to visible light. Another reason is that x-rays are reflected by and thus allow examination of structures having some characteristic size on the order of the wavelength of the x-ray used.
For very small structures, it is necessary to be able to tune an x-ray device very precisely, in terms of the average wavelength of radiation produced. This is because, in general, radiation is useful for investigating objects having a characteristic size on the order of the wavelength of the radiation. The smaller the characteristic size, the smaller the wavelength required. Thus, the precision with which the size can be established is important. Similarly, it is important that the radiation be composed of radiation having as few different wavelengths as possible. This aspect of radiation is referred to as its "chromatic" distribution. A substantially monochromatic distribution is highly useful.
It is also useful to be able to precisely direct the radiation. This is true for many reasons. For medical uses, it is known that radiation is harmful if large amounts are applied to any one spot, but much less harmful, if at all, if small doses are applied to that spot. Therefore, it is desirable to be able to prevent regions of the body not under investigation from being exposed to radiation. Further, a highly directional beam can be transformed by optical components--such as mirrors--to produce illuminated regions of desired shape and size. Little of the radiation need be wasted by such transformations.
In addition to using radiation to observe phenomena, radiation can be used to alter structures. For instance, light energy can burn through certain substances. Precisely pinpointed radiation of the right wavelength and power can etch substrates, change local chemical compositions, such as polymerization, etc.
A preferred embodiment of the disclosed invention is a compact embodiment, that can generate photons with energies freely adjustable between roughly 20 eV and 50 KeV. Apparatus with such capabilities has not previously been available. Many potential applications for such an apparatus are summarized in Table 2.
Since the invention of the laser in 1960, methods for generating monochromatic (i.e. having a narrow spread of frequencies) beams of electromagnetic radiation at soft x-ray frequencies and higher (corresponding to photon energies .gtoreq.100 ev) have been proposed by many researchers. The more extensively studied ideas include: the x-ray laser (using atomic and molecular transitions), Raymond C. Elton, X-Ray Lasers (Academic Press, New York, 1990); the .gamma.-ray laser (using nuclear transitions), Gagan Gupta and Javed Husain, "Prospects of Gamma-Ray Laser Development", Mod. Phys. Lett. B 5, 915-922 (1991); the x-ray free electron laser (FEL), J. Gea-Banacloche, G. T. Moore, R. R. Schlicher, M. O. Scully and H. Walther, "Soft X-Ray Free-Electron Laser With a Laser Undulator", IEEE J. Ouant. Electr. QE-23, 1558-1570 (September 1987) and Peter Dobiasch, Pierre Meystre and Marlan O. Scully, "Optical Wiggler Free-Electron X-Ray Laser in the 5 .ANG. Region", IEEE J. Ouant. Electr. QE-19, 1812-1820 (December 1983); channeling through crystals, J. O. R. H. Pantell, S. Datz, R. K. Klein and H. Park, "Thermal-Vibrational Amplitudes of Silicon Determined by Channeling-Radiation Measurements", Phys. Rev. B 44, 1992-2002 (1991) and Richard A. Carrigan, Jr. and James A. Ellison, eds., Relativistic Channeling (Plenum, New York, 1987); and transition radiation, M. A. Piestrup, D. G. Boyers, C. I. Pincus, J. L. Harris, X. K. Maruyama, J. C. Bergstrom, H. S. Caplan, R. M. Silzer and D. M. Skopik, "Quasimmochromatic X-Ray Source Using Photoabsorption-Edge Transition Radiation", Phys. Rev. A 43, 3653-3661 (1991) and M. A. Piestrup, M. J. Moran, D. G. Boyers, C. I. Pincus, J. O. Kephart, R. A. Gearhart and X. K. Maruyama, "Generation of Hard X-Rays from Transition Radiation Using High-Density Foils and Moderate-Energy Electrons", Phys. Rev. A 43, 2387-2396 (1991). More speculative ideas include: inverse Compton scattering from interfering laser beams, M. Bertolotti and C. Sibilia, "Coherent .gamma.-Ray Production", J. Sov. Laser Res. 6, 492-495 (1985) and M. Bertolotti and C. Sibilia, "Coherent .gamma. Radiation Production by Interaction Between a Relativistic Electron Beam and Two Interfering Laser Fields", Phys. Rev. A 26, 3187-3197 (December 1982); the induced annihilation of para-positronium atoms, Daniel M. Heffernan and Richard L. Liboff, "Induced Decay of Positronium and Grasers", Int. J. Theor. Phys. 22, 193-206 (1983); and the relativistic electron-positron "superpinch" of Winterberg, F. Winterberg, "Relativistic Electron-Positron Gamma Ray Laser", Z. Naturforsch, 41a, 1005-1008 (1986).
To date, none of the ideas proposed have theoretically been able to realistically generate high energy radiation in the desired range. For instance x-ray lasers have not been tunable to selected frequncies. Further, only relatively low energies have been achieved. Lasers for generating .gamma.-rays are also not tunable, and none have been developed that have approached workability. Free electron lasers for generating x-rays require mirrors that are sufficiently reflective at the wavelength of x-rays that are to be emitted. Such mirrors have not yet been developed. These have not been demonstrated to be practical to date. Generating radiation by transition radiation does not produce monochromatic or tunable output. It requires accelerators with electron energy E.gtoreq.200 MeV. Channeling radiation has an output power that is limited by heat deposited by an electron beam in the crystal, which is the source of the radiation.